The impact of bacteria-derived ultrafine dust particles on...

Yang et al. Experimental & Molecular Medicinehttps://doi.org/10.1038/s12276-019-0367-3 Experimental & Molecular Medicine

REV I EW ART ICLE Open Ac ce s s

The impact of bacteria-derived ultrafine dustparticles on pulmonary diseasesJinho Yang 1,2, Eun Kyoung Kim1, Hyeon Ju Park1, Andrea McDowell1 and Yoon-Keun Kim1

AbstractThe relationship between ambient particulate matter exposure and health has been well established. Ultrafine particles(UFP) with a diameter of 100 nm or less are known to increase pulmonary disease risk. Biological factors in dustcontaining UFP can cause severe inflammatory reactions. Pulmonary diseases develop primarily as a result of chronicinflammation caused by immune dysfunction. Thus, this review focuses on the adverse pulmonary effects of biologicalUFP, principally lipopolysaccharide (LPS), and bacterial extracellular vesicles (EVs), in indoor dust and thepathophysiological mechanisms involved in the development of chronic pulmonary diseases. The impact of LPS-induced pulmonary inflammation is based primarily on the amount of inhaled LPS. When relatively low levels of LPSare inhaled, a cascade of immune responses leads to Th2 cell induction, and IL-5 and IL-13 released by Th2 cellscontributes to asthma development. Conversely, exposure to high levels of LPS induces a Th17 cell response, leadingto increased production of IL-17, which is associated with asthma, COPD, and lung cancer incidence. Responses tobacterial EV exposure can similarly be broadly divided based on whether one of two mechanisms, either intracellularor extracellular, is activated, which depends on the type of the parent cell. Extracellular bacteria-derived EVs can causeneutrophilic inflammation via Th17 cell induction, which is associated with asthma, emphysema, COPD, and lungcancer. On the other hand, intracellular bacteria-derived EVs lead to mononuclear inflammation via Th1 cell induction,which increases the risk of emphysema. In conclusion, future measures should focus on the overall reduction of LPSsources in addition to the improvement of the balance of inhaled bacterial EVs in the indoor environment to minimizepulmonary disease risk.

IntroductionParticulate matterThe air we breathe inside our homes, workplaces,

schools, and public spaces is ubiquitously contaminatedwith a variety of types of particulate matter that compriseour indoor environment. Epidemiological studies world-wide have consistently demonstrated links betweenambient particulate matter exposure and adverse healthoutcomes, including increased rates of respiratory andcardiovascular illness, hospitalization, and prematuremortality1–3. Particles are usually defined based on theirsize: coarse particles are those with a diameter of 10 μm orless (PM10), fine particles are those with a diameter of

2.5 μm or less (PM2.5), and ultrafine particles (UFP) arethose with a diameter of 100 nm or less, which areabundant in number but make only a minor contributionto the total mass4. UFP do not readily sediment or floc-culate and thus have a longer retention time in theatmosphere than fine particles, allowing them to be car-ried long distances by the wind5,6. In terms of healthhazards, fine particles are only absorbed by alveolarmacrophages when they enter the lungs, whereas UFP arealso absorbed by airway epithelial cells, which can triggera potent airway inflammatory response. Moreover, unlikefine particles, UFPs are absorbed into the body andincrease the risk of cardiovascular and neuropsychiatricdisease. Due to this ability to more deeply infiltrate ournormal lines of defense against foreign particulate matter,UFP are likely to be more harmful to health than fineparticles5–7.

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Correspondence: Yoon-Keun Kim ([email protected])1Institute of MD Healthcare Inc., Seoul, Republic of Korea2Department of Health and Safety Convergence Science, Graduate School ofKorea University, Seoul, Republic of Korea

Official journal of the Korean Society for Biochemistry and Molecular Biology

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In addition, since most people spend ~87% of their timeindoors8, interest in the correlation between indoorenvironments and health has been increasing9,10. Previousstudies have suggested that indoor dust is a moreimportant factor in health and disease than outdoor dust.Indoor dust has particularly significant effects on health,as it includes a wide variety of components, includingPM10, PM2.5, nanoparticles, heavy metals, bacteria, fungi,and house dust mite-derived allergens. However, promi-nent organizations such as the World Health Organiza-tion (WHO) and Environmental Protection Agency (EPA)have not released specific regulations or guidelines interms of the particular composition of pollutants ornanosized particles in the indoor environment. Rather, theexisting regulations more broadly address the total bac-teria count (TBC) and the acceptable levels of PM10,PM2.5, heavy metals, gaseous pollutants and other pollu-tants. Therefore, recent efforts have focused on exploringthe specific causative factors and mechanisms involved inthe effects of UFP as indoor air pollutants to understandthe correlation between indoor air dust and disease.

Impact of the biological factors in indoor dust on healthInhaled air pollutants are correlated with pulmonary

disease and allergic diseases influenced by the environment,

such as atopic dermatitis, asthma, and allergic rhinitis.Aside from air pollutants, smoking is also known to be animportant factor in the development of chronic inflamma-tory pulmonary diseases such as asthma, chronic obstruc-tive pulmonary disease (COPD), and lung cancer11,12.Furthermore, exposure to chemicals with harmful phar-macological effects is also a potential contributing factor tochronic inflammatory disease. However, the etiology of alarge proportion of chronic inflammatory diseases in thelungs cannot be explained by the effects of such chemicalfactors alone. In addition, many COPD and lung cancerpatients have no history of cigarette smoking13. Interest-ingly, the relationship between serum anti-dust extracellularvesicle (EV) IgG levels and the risk of asthma, COPD, andlung cancer is stronger than that for cigarette smoking14.Pulmonary diseases are a result of chronic inflammationinduced by immune dysfunction stemming from the pre-sence of genes conferring susceptibility and environmentalfactors, including chemical substances and biological fac-tors. Asthma, COPD, and lung cancer generally develop as aresult of pathology induced by chronic inflammation, e.g.,airway hyperreactivity, airway fibrosis, alveolar destruction,cell dysplasia, and tissue invasion (Fig. 1a).Biological factors such as allergens, viruses, and bac-

terial substances in dust can cause severe inflammatory

Fig. 1 Asthma, COPD, and lung cancer development is associated with chronic inflammation, which is a result of immune dysfunction.Immune dysfunction is induced by susceptible genes and environmental factors, such as chemical and biological substances. Biological factorsincluding allergens and biological proteins that act as antigens induce Th1, Th2, and Th17 cell immune responses. These immune responses lead tomononuclear, eosinophilic and neutrophilic inflammation and contribute to the development of COPD, asthma, and lung cancer.

Yang et al. Experimental & Molecular Medicine of 10


reactions in certain cases. For example, bacterial factorsmight induce airway hypersensitivity due to exposure toprotein antigens. Even when humans are exposed to verysmall amounts of bacterial factors, the body recognizesthe antigens as harmful, and this can cause an inflam-matory reaction. Clinical manifestations may varydepending on the type of inflammation that occurs in theairways15–17. Biological factors in indoor dust can induceimmune dysfunction and chronic inflammation, leadingto chronic inflammatory pulmonary diseases, includingasthma and COPD18,19. Biological factors acting as anti-gens can differentially induce Th1, Th2, and Th17 cellimmune responses, leading to the release of IFN-gamma,IL-5/IL-13, and IL-17; subsequently, these immuneresponse factors lead to the activation of the mono-nuclear, eosinophilic, and neutrophilic pathways, respec-tively, and contribute to the development of COPD,asthma, and lung cancer (Fig. 1b).Previous studies have demonstrated that Th17 is

important for pathogen clearance and the support of hostdefense reactions and induces tissue inflammation inautoimmune diseases20. Functionally, angiogenesis andtumor growth are promoted by the overexpression of IL-17 in tumor cell lines in immune-compromised mice21. Inaddition, lung metastasis was shown to be suppressed byIL-17A deficiency or IL-17A blockade in tumor models22.Furthermore, IL-17 could directly promote the invasion ofnon-small cell lung cancer (NSCLC) cells, and an elevatedIL-17 level in peripheral blood was shown to be related tothe tumor-node-metastasis (TNM) stage23. These resultsindicate that tumor development is promoted by IL-17-mediated responses through the production of a micro-environment associated with tumor promotion and that aprimary mechanism of tumor induction is the IL-17-mediated regulation of myeloid-derived suppressor cells(MDSCs)24. Regarding Th17 cells in lung cancer, recentstudies have supported the role of Th17 cells in lungcancer pathogenesis. The levels of Th17 markers, such asRORC2, RORα4, IL-17A, IL-17A receptor, and VEGF,were significantly elevated in lung cancer patients25,26,and immune effector T cells (Teff) from small cell lungcancer (SCLC) patients included a large population ofTh17 cells27. Furthermore, CD4+ -IL-17A+ andCD8+ -IL-17A+ cells were detected in lung adeno-carcinoma tissues, whereas they were absent in normaltissues, suggesting the pathogenic role of IL-17A in lungadenocarcinoma26. Intranasal treatment with a neutraliz-ing anti-IL-17A antibody in a lung adenocarcinomamodel was also shown to reduce tumor growth25.

Biological UFP in indoor dustBiological fine particles contain microorganisms (bac-

teria and fungi) and contribute to ~5–34% of indoor airpollution28. However, a concrete scientific explanation of

the underlying mechanisms and the extent to whichindoor contaminants, including UFP, cause diseaseremains elusive. Microbes themselves cannot be classifiedas biological UFP, as the typical bacterium is over amicrometer in diameter, and pathogenic bacteria actprimarily as infectious agents. However, nanometer-sizedsubstances derived from microbes, such as lipopoly-saccharide (LPS) and EVs, are common UFP in indoordust. LPS has been characterized as having various sizesthat are in the range of a few hundred nanometers;however, there have been numerous reports of nanosized(10–200 nm) LPS derived from bacteria29–31. BacterialEVs are lipid bilayer vesicles that act as cell-to-cell com-municators between bacterial cells or eukaryotic cells andare secreted during cell proliferation and death32,33. Theendotoxin LPS is a cell wall component of gram-negativebacteria that is ubiquitous in our living environment34.LPS, which directly affects the development of disease, isboth embedded in the cell membrane of gram-negativeEVs and detached from the cell membrane in the solublefraction (SF). The amount of LPS in dust SF was greaterthan that in dust EV; however, the levels of TNF- α andIL-6 induced by dust EV were significantly higher thanthose induced by dust SF35. Thus, this review focuses onthe adverse pulmonary effects of biological UFP in indoordust, such as LPS and bacterial EVs, and the pathophy-siological mechanisms involved in the development ofchronic pulmonary diseases such as asthma, COPD, andlung cancer.

Lipopolysaccharide as a biological UFPAssociation between LPS and pulmonary diseases basedon epidemiological studiesThe endotoxin LPS is a cell wall component of gram-

negative bacteria that is ubiquitous in our living envir-onment. LPS is a contaminant in various organic dustsand environments that support gram-negative bacterialgrowth. Exposure to LPS induces the production of pro-inflammatory and immune-modulating factors viaTLR434,36. Furthermore, LPS is known to be a biologicalfactor commonly derived from bacteria that is associatedwith chronic inflammatory pulmonary disease13. A sig-nificant and growing body of literature has reported on theassociation between endotoxin exposure and pulmonarydisease, including asthma, COPD, and emphysema37.Airborne endotoxin exposure has been linked with

occupational respiratory diseases in work environmentsassociated with high concentrations of organic dust, suchas agricultural sites, cotton mills, breweries, and wasteprocessing sites38. Interestingly, it has been reported thatthe LPS concentration in dust rather than the dust itself ispositively correlated with decreased pulmonary func-tion39. Epidemiological studies suggest that endotoxinexposure is positively associated with an increased risk of



asthma and asthma severity in both adults and children40.Furthermore, atopic asthma has also been positivelyassociated with early childhood exposure to endotoxinsthrough the transmission of infectious disease41. Similarly,a higher abundance of airway microbiota and increasedLPS levels have been identified in patients with neu-trophilic asthma42. Altogether, these epidemiologicalfindings suggest that exposure to LPS via inhalation maybe positively correlated with respiratory disease.

Pathophysiological mechanisms of LPS involved in thedevelopment of pulmonary diseaseAlthough efforts to determine the concrete etiological

basis of pulmonary disease are ongoing, several studieshave confirmed that LPS is a causative factor in respira-tory disease and other chronic inflammatory illnesses. In amouse model, an allergen challenge was performed inthree ways: ovalbumin (OVA) with no LPS, OVA with0.1 µg of LPS, or OVA with 100 µg of LPS. While treat-ment with OVA, a protein commonly used to stimulateallergic reactions, did not induce inflammatory responses,OVA combined with LPS was reported to induceinflammatory responses, suggesting that LPS is anessential element of the inflammatory response associatedwith immune dysfunction in the airways. Moreover, OVAcombined with a low dose of LPS led to Th2 cellresponses, and OVA combined with a high dose of LPSwas shown to induce Th1 inflammatory responses. Thesefindings confirmed that the inflammatory immuneresponse in the airway is differentially induced dependingon the amount of LPS present in the indoor air environ-ment43. Our group’s research findings also demonstratedthat different forms of asthma can be induced in anin vivo mouse model depending on the amount of LPS.Animal models indicated that allergen sensitization with alow dose of LPS (0.1 µg) induced type 2 asthma pheno-types, such as airway hyperresponsiveness, eosinophilicinflammation, and allergen-specific IgE upregulation.Allergen sensitization with 10 µg of LPS-induced asthmaphenotypes such as airway hyperresponsiveness and Th17cell-mediated neutrophilic inflammation18.Specific immunological responses have been shown to

vary in response to differing levels of LPS in the airway,which can be generally classified according to twoimmunological mechanisms: Th2 and Th17 immuneresponses (Fig. 2). It has been well established that lowdoses of inhaled LPS induce a Th2 cell response. Fur-thermore, when a combination of a low dose of LPS andallergens enters the airway through inhalation, only LPS isabsorbed into airway epithelial cells and immune cells,such as macrophages, natural killer T (NKT) cells, anddendritic cells (DCs). Airway epithelial cells activated by alow dose of LPS induce the secretion of pro-inflammatorymediators, such as thymic stromal lymphopoietin (TSLP).

TSLPs derived from epithelial cells both function in DCactivation and enable DCs to polarize naïve T cells,leading to the production of proallergic Th2 cytokinessuch as IL-5 and IL-1344. Furthermore, this immunolo-gical pathway leads to the production of (TNF-α) insurrounding cells in response to macrophages andNKT cells activated by TSLP or low levels of LPS. TNF-αreleased by the surrounding immature dendritic cells theninduces the increased expression of costimulatory mole-cules and IL-4 in dendritic cells18. These mature dendriticcells then migrate to the draining lymph nodes (DLN) andinduce T cell proliferation and differentiation into Th2cells through the production of IL-4, activating the signaltransducer and activator of transcription 6 (STAT6) sig-naling pathway45. Through the STAT6 pathway, chronicinflammatory signaling is initiated as Th2 cells areinduced to release IL-5 and IL-13, which are pro-inflammatory cytokines associated with the developmentof eosinophilic asthma and eosinophilic bronchitis.Conversely, exposure to high levels of inhaled LPS is

primarily associated with the Th17 cell response. High-dose LPS exposure induces vascular endothelial growthfactor (VEGF) release from surrounding cells, includingairway epithelial cells, macrophages, and NKT cells46.VEGF is produced during the LPS-induced innateresponse and acts as a potent mediator of vascularremodeling and angiogenesis in the lungs47. VEGF indu-ces DC maturation and expression of costimulatorymolecules via the vascular endothelial growth factorreceptor 1 (VEGFR1)-dependent pathway. Through thispathway, IL-6-expressing DCs are then upregulated, andmature DCs are transferred to the DLN. Upon T cellproliferation, the excess T cells differentiate into Th17cells via the IL-6 and STAT3 signaling pathways. Throughthe immunological response to high levels of airborneLPS, Th17 cells are induced to secrete IL-17, contributingto the development of serious pulmonary diseases such asneutrophilic asthma, COPD, and lung cancer18,46,47.

Bacterial EVs are a key etiological agent of chronicpulmonary inflammatory diseasesBacterial EVs are vesicles encased in a lipid bilayer

containing transmembrane proteins, cytosolic proteins,LPS, and nucleic acids that range from 20 to 100 nm indiameter. EVs are ubiquitous in all bacteria and arecommonly referred to as outer membrane vesicles(OMVs) in Gram-negative bacteria and membrane vesi-cles (MV) in Gram-positive bacteria32,33. After oraladministration of bacteria-derived EVs, the EVs areabsorbed through the intestinal barriers and transportedto various organ systems, including the liver, adiposetissue and skeletal muscle48. As the EV membrane isembedded with surface ligands that interact with recep-tors on target cells, EVs can attach to and modify the



Fig. 2 Low level-lipopolysaccharide (LPS) in the airway is absorbed into airway epithelial cells and immune cells, such as macrophages,natural killer T (NKT) cells, and dendritic cells (DCs). Exposure to low levels of LPS in the airway is associated with Th2 immune response and Th2cytokines, such as IL-5 and IL-13, which contribute to the development of asthma. High levels of LPS in the airway induce vascular endothelial growthfactor (VEGF). High levels of LPS is related with Th17 immune response and increased Th17 cytokines, such as IL-17, that contributes to thedevelopment of asthma, COPD, and lung cancer.



physiological state of recipient cells49,50. Furthermore,bacterial EVs have recently been shown to be involved inthe development of a wide variety of diseases, includingcancer, diabetes, atopic dermatitis, and meningitis.

Association between EVs and pulmonary diseases basedon epidemiological studiesA variety of epidemiological evidence has suggested a

strong link between the presence of bacterial EVs in dustin the indoor environment and chronic pulmonary illness.For example, a previous clinical study reported that whileonly ~5% of patients with allergic rhinitis or atopic der-matitis and healthy subjects showed sensitization to bac-terial EVs present in apartment dust, over 50% of patientswith childhood asthma were shown to be sensitized toEVs in indoor dust. This finding indicates the particularlysignificant role that EVs in indoor dust play in thedevelopment of childhood asthma-associated immuneresponses35. Higher serum anti-dust EV IgG antibodylevels were also reported in patients with noneosinophilicasthma, COPD, and lung cancer than in healthy con-trols14. Interestingly, adjusted multiple logistic regressionshowed that the significant increase in anti-dust EV IgGantibody levels in patients with pulmonary disease com-pared with those in healthy controls was an independentrisk factor for asthma, COPD and lung cancer irrespectiveof more typically associated factors, such as smokinghistory, age, and sex14. Altogether, while research on theimpact of bacterial EVs in indoor dust on pulmonarydisease is in its early stages, preliminary findings indicatethat bacterial EVs pose a serious risk in terms of asthma,COPD, and lung cancer incidence.

Pathophysiological mechanism of microbial EVs involvedin the development of pulmonary diseaseOur group was the first to report the presence of large

amounts of bacterial EVs in indoor dust collected frombeds in apartments32,51,52. Significantly, these indoor dustbacterial EVs were found to be mainly derived frompathogenic bacteria. The microbial EV composition ofindoor dust is dominated by bacteria from the generaPseudomonas, Acinetobacter, Enterobacter, and Staphy-lococcus (Manuscript in submission). We also demon-strated that prolonged exposure to bacterial EVs ininhaled indoor dust induces significant airway inflam-mation that leads to severe asthma-like responses as wellas emphysema. The induction of emphysema is of parti-cular concern, as it is known to be a major factor in thedevelopment of irreversible airway obstruction, includingCOPD35.Airway exposure to microbial EVs triggers two main

pathophysiological mechanisms based on whether theparent cell is extracellular vs. intracellular: the Th17response is induced in response to extracellular bacteria-

derived EVs, especially Gram-negative OMVs, and theTh1 response is induced in response to intracellularbacteria-derived EVs, such as Gram-positive myco-bacterial MVs (Fig. 3). Specifically, extracellular bacterialEVs generally induce neutrophilic inflammation via IL-17release by polarized Th17 cells, while intracellular bac-terial EVs cause mononuclear inflammation through IFN-γ produced by polarized Th1 cells. The neutrophilicinflammatory response induced by pathogenic bacterialEVs generally leads to airway hyperreactivity and fibrosisthat contributes to asthma development, while the com-bination of increased elastase production and fibrosisinduced by neutrophilic inflammation can lead to COPD.Furthermore, epithelial cell dysplasia and induction ofmatrix metalloproteinase (MMP) expression associatedwith neutrophilic inflammation can lead to lung cancer,and an increase in COPD incidence increases lung cancerrisk. Meanwhile, intracellular bacterial EVs, such asmycobacterial EVs, lead to Th1 polarization and sub-sequent IFN-γ-induced mononuclear inflammation,leading to increased elastase production in alveoli andthus causing emphysema.Our previous studies have shown that repeated airway

exposure over four weeks to Gram-negative Escherichiacoli (E. coli)-derived EVs in mice induced IL-17A-dependent neutrophilic inflammation and emphysema inconjunction with elastase upregulation53. Intraperitonealinjection of EVs derived from intestinal E. coli inducedhost responses that resemble clinically relevant condi-tions, such as systemic inflammatory response syndrome(SIRS), that were characterized by piloerection, eye exu-dates, hypothermia, tachypnea, leukopenia, disseminatedintravascular coagulation, dysfunction of the lungs,hypotension, systemic increases in TNF-a and IL-6 pro-duction, and lethality54. Pseudomonas aeruginosa (P.aeruginosa) often contributes to pulmonary diseases suchas cystic fibrosis, and its EVs increase pulmonaryinflammation through activation of the TLR2 and TLR4pathways. P. aeruginosa-derived EVs induce pulmonaryinflammation via dose-dependent increases in the che-mokines CXCL1 and CCL2 and the cytokines IL-1β,TNF-α, IL-6, and IFN-γ and increases in neutrophils andmacrophages in vivo55. Moreover, indoor dust, whichcontains many components of bacteria, has been asso-ciated with both Th1 and Th17 responses, which induceneutrophilic pulmonary inflammation35,56. As bacteria-derived EVs can affect distal host cell sites14, these vesiclesmight have significant effects throughout the body. Theseresults suggest that the pathogenicity of bacterial EVs isdependent on the substances contained in the EVs.Although Gram-negative OMVs are better understood

and characterized than Gram-positive MVs, severalstudies have uncovered immunological responses tosome common Gram-positive MVs in the airway. For



example, mononuclear cell-induced inflammatoryresponses associated with Gram-positive EVs producedby pathogens such as Staphylococcus aureus (S. aureus),a typical gram-positive bacterium, have been implicatedin atopic dermatitis-like skin inflammation51,52. S. aur-eus EVs increase IL-6, IL-17, serum IgG1, IgE, TSLP,MIP-1α, and eotaxin levels in addition to increasing therecruitment of mast cells and eosinophils in the skin51.Furthermore, repeated exposure to S. aureus EVs in theairways induced both Th1 (IFN-γ) and Th17 (IL-17)immune responses in addition to increasing neutrophilicpulmonary inflammation primarily through TLR2engagement57. While the immune response to airway

microbial EVs can be broadly categorized based on theintracellular vs. extracellular type of the original cells,the specific immune responses to different types ofbacterial EVs are summarized in Table 1. While variousimmune responses have been reported for differentspecies of microbial EVs, common airway immuneresponses can be identified based on the Gram-type ofindoor dust microbial EVs. For example, common airwayGram-negative EVs, such as those derived from P. aer-uginosa, E. coli, and Acinetobacter baumanii (A. bau-manii), all increased IL-6 levels and neutrophilic activity.Meanwhile, Gram-positive EVs, such as those derivedfrom S. aureus and Faecalibacterium prausnitzii

Fig. 3 Bacteria-derived extracellular vesicles (EVs), especially Gram-negative outer membrane vesicles (OMVs), induce Th17 immuneresponse and lead to neutrophilic inflammation. Intracellular bacteria-derived EVs, such as Gram-positive mycobacterial membrane vesicles (MVs)induce Th1 response and lead to mononuclear inflammation. These immune responses contribute to the development of asthma, emphysema,COPD, and lung cancer.



(F. prausnitzii), have been reported to commonlyincrease IFN- γ levels.

ConclusionProlonged exposure to biological UFP contained in

indoor dust has been recently demonstrated to induce avariety of innate and chronic immune responses that canlead to the development of a variety of respiratory dis-eases. Generally, bacterial UFP can be segregated into twoclasses: very fine LPS particles and bacterial EVs. Pre-viously, LPS has been targeted as the key biological UFPcontributing to indoor dust, and recent studies havehighlighted the influence of bacterial EVs. For example, ithas been shown that treatment with Gram-negative EVselicits stronger pro-inflammatory immune responses thanthat with isolated LPS, primarily due to the necessity ofvesicle proteins for macrophage internalization58. Suchfindings suggest that while Gram-negative bacterialOMVs contain LPS, other components of bacterial EVsinfluence the activation of pro-inflammatory innateimmune responses by the innate immune system. Fur-thermore, EVs from Gram-positive bacteria lacking LPSare similarly able to differentially illicit both powerful pro-and anti-inflammatory immune responses, emphasizing

the complex effects bacterial EVs have on chronic pul-monary disease risk.As understanding of the significant health risks posed

by inhaled ultrafine biological particles begins to coalescewithin the scientific community, it is necessary toimplement appropriate measures to improve the airquality of indoor environments to reduce chronic pul-monary disease risk. Although LPS as an isolated ligandplays a more straightforward dose-dependent etiologicalrole in pulmonary disease, the complex, dynamic natureof bacterial EVs requires a more holistic approachtowards the optimization of the indoor environment.While further metagenomic and proteomic assessmentof bacterial EVs in a wider variety of indoor environ-ments and vulnerable populations is required, pre-liminary findings indicate that efforts to improve theindoor dust bacterial EV balance should focus on thenormalization of environments with heavy amounts ofGram-negative contamination. In conclusion, futuremeasures should focus on the overall reduction of LPSsources in addition to the improvement of the overallbalance of inhaled pathogenic and beneficial bacterialEVs in the indoor environment to minimize pulmonarydisease risk.

Table 1 Immune responses to bacterial extracellular vesicles.

Bacterial EV Immune response Reference

Gram Negative OMV

Pseudomonas aeruginosa Dose-dependent gene and protein expression of MIP-2, TNF-a, IL-1B, and IL-6 in vitro. Ellis, et al.58

Induction of pulmonary inflammation via dose-dependent increases in CXCL1, CCL2, IL-1β, TNF-α, IL-6, and

IFN-γ as well as increases in neutrophils and macrophages in vivo.

Park, et al.55

Escherichia coli Induction of systemic inflammatory responses via increases in serum TNF-α, IL-6, and IFN-γ and increases

in lung permeability and dysfunction in vivo.

Park, et al.54

Induction of emphysema through IL-17a-mediated neutrophilic inflammation in vivo. Kim, et al.53

Induction of systemic and lung inflammation via increased TNF-α and IL-6 levels in serum and

bronchoalveolar lavage fluid in vivo.

Jang, et al.59

Acinetobacter baumannii Intratracheal administration of EVs elicits expression of IL-1β, IL-6, IL-8, MIP-1α, and MCP-1 in lungs in

addition to vacuolization, detachment of epithelial cells and neutrophilic infiltration.

Jun, et al.60

Hemorrhage, necrosis, and infiltration of polymorphonuclear leukocytes in lung tissue 48 h after

intratracheal infection of EVs in vivo.

Jin, et al.61

Gram Positive MV

Staphylococcus aureus Increases in IL-6, IL-17, serum IgG1, IgE, TSLP, MIP-1α, and eotaxin levels in addition to the recruitment of

mast cells and eosinophils in a dose-dependent manner.

Hong, et al.51

Induction of neutrophilic pulmonary inflammation through Th1 and Th17 cell responses in vivo. Kim, et al.57

Faecalibacterium prausnitzii Upregulation of the anti-inflammatory cytokines IL-10, TFG-β2, and IL-1RA and downregulation of the pro-

inflammatory cytokines IL-6, TNF-α and TNF-β in a lung cancer cell line.

Jafari, et al.62

OMV outer membrane vesicles, MIP macrophage inflammatory proteins, TNF tumor necrosis factor, IL interleukin, CXCL chemokine (C-X-C motif) ligand, CCLChemokine (C-C motif) ligand, MV membrane vesicles, Ig immunoglobin, s-Ig secretory Ig, IFN Interferon, TSLP thymic stromal lymphopoietin, TFG transforminggrowth factor



AcknowledgementsThis research was supported by the Bio & Medical Technology DevelopmentProgram of the National Research Foundation (NRF) funded by the Koreangovernment (MSIT) (NRF-2016M3A9B6901516).

Conflict of interestThe authors declare that they have no conflict of interest.

Publisher’s noteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.

Received: 6 September 2019 Revised: 6 November 2019 Accepted: 14November 2019

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